This invention relates to an electrostatic printing apparatus and in particular to a developing apparatus utilizing a magnetic brush.
A developing apparatus is already known which utilizes a magnetic brush to clean a photosensitive layer of a cylindrical drum or to develop an electrostatic latent charge image formed on the photosensitive layer of the cylindrical drum. An electrostatic printing apparatus equipped with a developing apparatus is manufactured and put on sale. However, it is generally pointed out that such conventional apparatus have a disadvantage as set out below. That is, when a paper sheet is copied on the conventional apparatus, a black-band stain or mark appears on the copied paper sheet. As a result of investigation it has been found that such black-band stain or mark occurs for the reason set out below.
When the cylindrical drum is at rest, a magnetic brush is contacted with the photosensitive layer of the magnetic drum and some toner powders or particles are attached to the contacted area of the photosensitive layer of the magnetic drum. Such toner powders remain there, while the magnetic drum continues to be rotated, causing a black-band smear to occur on a copied paper sheet. It has been found that such smear can be partially eliminated by electrostatically attracting such attached toner powders away from the photosensitive layer of the magnetic drum. It has also been found that the smear can not completely removed merely by such an electrostatic attraction method. As a result of further investigation it has been found that when the developing apparatus starts to be rotated, i.e. the magnetic roller starts to be rotated, some toner powders fly off onto the photosensitive layer of the cylindrical drum from near the doctor blade due to vibrations and they are deposited there. A demand is, therefore, made for a means for preventing some toner powders from flying off onto the photosensitive layer of the cylindrical drum from near the doctor blade.